Certain types of glasses and glass-ceramics can be chemically strengthened by an ion exchange process that may change the surface refractive index of the material. The strengthening is due to the formation of a near-surface compression layer that usually creates birefringence.
With the increased use of chemically strengthened glasses and glass-ceramics in such products as smart phones, computer screens and flat-panel televisions, there is an increasing need for nondestructive, high-throughput measurements of the surface stress for quality control during manufacturing.
A nondestructive prism-coupling-based method for characterization of the stress profile is currently used for chemically strengthened glasses in which the ion-exchange process results in an increase in the surface refractive index and thus the formation of an optical waveguide. The method is based on index-profile recovery for two orthogonal polarizations of visible-wavelength light based on measurements of the transverse-electric (TE) and transverse-magnetic (TM) guided mode spectra. The difference in the two index profiles (namely, in the depth distribution of birefringence) is approximately linearly related to the depth distribution of stress via the stress-optic coefficient (SOC).
Another prism-coupling-based method is used for measuring the surface refractive index of transparent substrates with a typical precision of ±0.0002 RIU (refractive index units). The method scans a laser beam over a range of angles at the prism-sample interface. If used to measure surface birefringence and stress, the method would produce a birefringence error of ±0.0003 RIU based on the summation rule for two random errors, one for each polarization measurement. Since the typical SOC of common chemically strengthened glasses is on the order of 3×10−6 RIU/MPa, the resulting stress-measurement error would be about 100 MPa. This is too large for most practical applications, where the typical surface stresses are on the order of 200-900 MPa. A measurement error below 5% is usually needed for process and quality control during manufacturing. In addition, the scanning used in this approach is time-consuming and results in slow measurements, which is not ideal for performing quality control in a commercial setting.
Another problem with conventional coupling prism-based stress measurement methods and systems is that they are not capable of making accurate measurements of surface stress in ion-exchange glasses with either a decreased surface refractive index or a high attenuation.